When preparing OLED devices usually printing techniques like inkjet printing, roll to roll printing, slot dye coating or gravure printing are used to apply the active layer. Based on low solubility of the most of the present organic compounds useful as emitting materials and/or charge transport materials, these techniques need the use of solvents in high amounts.
In order to improve the film forming ability binding agents can be used. These additives are especially needed with regard to light emitting materials and/or charge transport materials having small molecular weight or polymeric compounds having a low molecular weight.
EP 1 883 124 A1 describes a formulation of light-emitting material particularly suitable for forming displays and lamps via printing techniques comprising organic-light emitting material housed in protective porous matrix material, a binder and a solvent. However, the OLED material encompasses also polymeric materials. Furthermore, the binder material is disclosed as a long list without any detailed specification.
US 2007/0103059 discloses compositions comprising an OLED material and a polymer having very specific repeating units. The polymer having specific repeating units is added to improve the emitting efficiency of the OLED. Also polymeric OLED materials can be employed.
According to U.S. Pat. No. 6,818,919 and U.S. Pat. No. 7,115,430 polymers having a high glass transition temperature Tg have to be used in order to process low molecular weight organic light emitting and charge transport materials. However, these materials are expensive and limit the application of the compositions.
U.S. Pat. No. 5,952,778 relates to an encapsulated organic light emitting device having an improved protective covering comprising a first layer of passivating metal, a second layer of an inorganic dielectric material and a third layer of polymer. The organic light emitting material can be polymeric or monomeric. The composition can contain a polymer binder. However, the binder material is disclosed in a long list without any detailed specification.
U.S. Pat. No. 6,277,504 B1 discloses specific light emitting compounds and compositions comprising the same. The compositions may include a binder. However, no detailed specification of the binder is provided.
U.S. Pat. No. 6,294,273 B1 describes light emitting compounds being soluble in methanol. The compositions comprising these compounds may contain polymer binders. However, the binder material is disclosed in a long list without any detailed specification.
WO 2005/055248 A2 relates to compositions comprising specific organic semiconductor compounds and an organic binder having a permittivity of 3.3 or less at 1000 Hz. However, the specific organic semiconductor compounds as disclosed in WO 2005/055248 A2 should form a layer having a high crystallinity in order to achieve a high efficiency. In contrast thereto, layers emitting light should usually have a low crystallinity for providing high efficiency. Therefore, the concept of WO 2005/055248 A2 cannot be applied to OLED layers.
Solutions of small molecules in a solvent do not generally have a big effect on the viscosity of the resultant solution as occurs with polymers. Therefore, compositions of small molecules for conventional printing applications like ink jet printing, flexographic or gravure printing need additives to increase ink viscosity and improve film formation. The prior art provides compositions being useful in order to process low molecular weight organic light emitting and charge transport materials. However, it is a permanent desire to improve the performance of the OLED layer, especially in terms of efficiency, lifetime and sensitivity regarding oxidation or water.
It is therefore desirable to have fluids comprising light emitting materials and/or charge transport materials that are suitable for the preparation of OLED devices by the aforementioned solution based processes, which allow the manufacture of very homogeneous OLED devices having a high efficiency, a long lifetime, and a low sensitivity against water or oxidation. One aim of the present invention is to provide such improved fluids. Another aim is to provide improved methods of preparing an OLED device from such fluids. Another aim is to provide improved OLED devices obtained from such fluids and methods. Further aims are immediately evident to the person skilled in the art from the following description.
Surprisingly it has been found that these aims can be achieved, and the above-mentioned problems can be solved, by providing methods, materials and devices as claimed in the present invention, especially by providing a process for preparing an OLED device using a composition comprising a polymeric binder.